6of9Interior Secretary Ryan Zinke speaks at the Interior Department in Washington, Wednesday, March 29, 2017. Zinke said geographic and physical challenges will make it difficult to build the "big, beautiful wall" that President Donald Trump has promised on the U.S.-Mexico border. (AP Photo/Molly Riley)Photo: Molly Riley, FRE

7of9Margaret McCall, a clean energy consultant from Chicago, sits in Santa Elena Canyon in the Rio Grande river facing a cliff that is Mexico, as she vacations at Big Bend National Park in Texas, Monday, March 27, 2017. When asked about the border wall, McCall said: "My first thought is: has Donald Trump seen this cliff? Because unless you're building a 500-foot wall it's really not going to cut it." (AP Photo/Rodrigo Abd)Photo: Rodrigo Abd, STF

8of9This photo taken on February 19, 2017 shows an aerial view of the metal fence between Mexico (L) and the United States in Puerto Anapra, Chihuahua state. Built to keep out migrants, traffickers, or an enemy group, border walls have emerged as a one-size-fits-all response to the vulnerability felt by many societies in today's globalized world, says an expert on the phenomenon. Practically non-existent at the end of World War II, by the time the Berlin Wall fell in 1989 the number of border walls across the globe had risen to 11. That number has since jumped to 70, prompted by an increased sense of insecurity following the September 11, 2001 attacks in the United States and the 2011 Arab Spring, according to Elisabeth Vallet, director of the Observatory of Geopolitics at the University of Quebec in Montreal (UQAM). This image is part of a photo package of 47 recent images to go with AFP story on walls, barriers and security fences around the world. More pictures available on afpforum.com / AFP PHOTO / YURI CORTEZYURI CORTEZ/AFP/Getty ImagesPhoto: YURI CORTEZ, Staff

9of9Interior Secretary Ryan Zinke speaks at the Interior Department in Washington, Wednesday, March 29, 2017. Zinke said geographic and physical challenges will make it difficult to build the "big, beautiful wall" that President Donald Trump has promised on the U.S.-Mexico border. (AP Photo/Molly Riley)Photo: Molly Riley, FRE

WASHINGTON - Geographic and physical challenges - including the Rio Grande and threatened wildlife - will make it difficult to build the "big, beautiful wall" that President Donald Trump has promised on the U.S.-Mexico border, Interior Secretary Ryan Zinke said Wednesday.

Building a wall "is complex in some areas," including Big Bend National Park and along the river, which twists through nearly half of the 2,000-mile border, Zinke said.

Hundreds of species live within 30 miles of the border, including threatened jaguars and Mexican gray wolves. The Trump administration is poised to relax protections for the jaguars, which live in northern Mexico and parts of the southwestern U.S., to make it easier to build the wall.

Throughout the campaign, Trump energized his crowds with his insistence that a wall will be constructed along the border and that Mexico will pay for it. Zinke's comments, and the administration's budget proposal seeking billions in U.S. taxpayer dollars to finance the project, offer a reality check and a possible sign that Trump is moving away from his initial plan.

The complications Zinke highlighted were the same faced by Trump's predecessors, George W. Bush and Barack Obama, as they sought to build or complete hundreds of miles of fencing along the border.

Fencing that is already in place is a mixture of various designs, including towering steel bollards designed to keep both people and vehicles from moving north and shorter steel posts aimed only at blocking cars. In parts of Texas' Rio Grande Valley, some stretches of fencing are nearly a mile away from the border in part to accommodate flood plains and an international treaty.

And in Texas, almost all of the land along the border is privately owned. When Bush tried to build border fencing starting in 2006, he faced stiff opposition from local ranchers and farmers, many of whom took the government to court on plans to use their land.

The Department of Homeland Security is responsible for the border wall, but Zinke said the Interior Department will play a critical support role. According to the Government Accountability Office, federal and tribal lands make up about 632 miles, or a third of the nearly 2,000-mile border.

"At the end of the day, what's important is American security and to make sure we have a border," Zinke told reporters. "Without a border, a nation cannot exist."

An internal report prepared for Homeland Security Secretary John Kelly estimated that a wall along the entire border would cost about $21 billion.

Republicans have estimated a more moderate price tag of $12 billion to $15 billion.

"The border is complicated, as far as building a physical wall," Zinke said earlier this week in remarks first reported by E&E News. "The Rio Grande, what side of the river are you going to put the wall? We're not going to put it on our side and cede the river to Mexico. And we're probably not going to put it in the middle of the river."

Electronic monitors may be more appropriate in that region, Zinke said, while areas with imposing natural features may not require additional reinforcements.

The border is already dotted with underground sensors and camera towers, along with about 700 miles of fencing in Texas, New Mexico, Arizona and California, and it's unclear how much new fencing the Trump administration is proposing.

According to new budget details sent to Congress, the administration wants immediate funding to complete an existing barrier in the Rio Grande Valley, $500 million to complete 28 miles of a border levee wall near McAllen and $350 million for construction along two segments near San Diego.

Senate Democrats have threatened to filibuster any provision providing money for the wall. Many Republicans aren't very enthusiastic about a wall.

Sen. Roy Blunt, R-Mo., a key budget negotiator, said the Senate is unlikely to include money for a border wall in a broader spending package to avert a partial government shutdown next month.